The role of modifications in codon discrimination by tRNALysUUU

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Abstract
The natural modification of specific nucleosides in many tRNAs is essential during decoding of mRNA by the ribosome. For example, tRNALysUUU requires the modification N6-threonylcarbamoyladenosine at position 37 (t6A37), adjacent and 3′ to the anticodon, to bind AAA in the A site of the ribosomal 30S subunit. Moreover, it can only bind both AAA and AAG lysine codons when doubly modified with t6A37 and either 5-methylaminomethyluridine or 2-thiouridine at the wobble position (mnm5U34 or s2U34). Here we report crystal structures of modified tRNA anticodon stem-loops bound to the 30S ribosomal subunit with lysine codons in the A site. These structures allow the rationalization of how modifications in the anticodon loop enable decoding of both lysine codons AAA and AAG.